Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to transmitting reference signals.
Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems, and single-carrier frequency division multiple access (SC-FDMA) systems.
These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. For example, a fifth generation (5G) wireless communications technology (which can be referred to as 5G new radio (5G NR)) is envisaged to expand and support diverse usage scenarios and applications with respect to current mobile network generations. In an aspect, 5G communications technology can include services such as: enhanced mobile broadband (eMBB) addressing human-centric use cases for access to multimedia content, services and data; ultra-reliable-low latency communications (URLLC) with certain specifications for latency and reliability; and massive machine type communications, which can allow a very large number of connected devices and transmission of a relatively low volume of non-delay-sensitive information. As the demand for mobile broadband access continues to increase, however, further improvements in 5G communications technology and beyond may be desired.
In long term evolution (LTE), for example, access points (e.g., Node B) transmit cell-specific reference signals (CRSs) to drive tracking loops at access nodes (e.g., user equipment (UE)). The CRSs allow for desirable processing gain and robust tracking performance by the UE to provide fine time tracking, fine frequency tracking, Doppler spread estimation and/or delay spread estimation. For example, the CRSs in LTE can span the entire system bandwidth (thus allowing fine time-domain resolution), have a ⅓ density after de-staggering (thus allowing desirable time-domain pull-in range), have guaranteed phase continuity, allow multiple observations per subframe (thus allowing desirable frequency domain pull-in range), are always transmitted by the access point, and can use 2-port transmission for spatial diversity. With the evolution of 5G, however, CRSs may no longer be practical as the always-on nature can introduce pilot pollution, leads to unnecessary energy consumption (e.g., when the network load is light), and/or gets in the way of flexible resource utilization and/or blanking.
Generally, in 5G, long term evolution (LTE), and/or other wireless communications, access points can also transmit channel state information reference signals (CSI-RSs) to user equipment (UE) for each of a plurality of antenna ports. The UEs can measure the CSI-RSs to determine channel characteristics (or channel state information (CSI)) for the corresponding antenna ports. Thus, the access point can also transmit a plurality of CSI-RS configurations to the UE to indicate resource elements (REs) over which CSI-RSs are transmitted for each of the corresponding antenna ports. With the sparse density of a given CSI-RS, however, a UE may not be able to acquire fine time/frequency tracking, Doppler spread estimation, delay spread estimation, etc. as previously attainable using CRSs in LTE.